1. Field of Invention
Embodiments of the present invention described herein generally relate to semiconductor devices and methods of manufacturing the same and, more particularly, to a sense amplifier and a method of forming the same.
2. Description of the Related Art
Generally, a semiconductor device is employed for storing data. A random-access memory (RAM) device is a kind of volatile memory device, and is mostly employed as a main memory device for a computer. A dynamic RAM (DRAM) device is a kind of RAM, and includes a unit memory cell having a transistor and a capacitor. A charge is either stored or not stored in the capacitor so that information having a value of “1” or “0,” respectively, may be written or read. Over time, charges stored in the capacitor may be lost. Thus, the capacitor included in the memory cell is refreshed in order not to lose the stored charge.
In a memory cell of a DRAM device including a transistor and a capacitor, a word line is connected to a gate structure of the transistor, and a bit line is connected to a source region of the transistor. The word line is enabled to turn on the gate structure so that data stored in the capacitor may be read through a bit line or data may be written in the capacitor through the bit line.
The data stored in a unit cell may be read by the following processes.
A bit line of the transistor is selected. A voltage of the selected bit line and a voltage of a bit line bar adjacent to the selected bit line are output. The output bit line voltage and the output bit line bar voltage are amplified to confirm whether the amplified bit line voltage is higher than the amplified bit line bar voltage or not. Thus, the data in the unit cell may be read using the confirmation result.
Generally, a bit line used in a DRAM device may have either a folded bit line structure or an open bit line structure. In the folded bit line structure, a bit line and a bit line bar are formed in parallel to be connected to a same side of a sense amplifier. In the open bit line structure, a bit line and a bit line bar that is comparable to the bit line are formed apart from each other so that the bit line and the bit line bar are connected to a sense amplifier at different sides of the sense amplifier. The open bit line structure may occupy an area smaller than the folded bit line structure, and thus the open bit line structure is employed for a bit line included in a recent semiconductor memory device having a high integration degree.
FIG. 1 is a plan view illustrating a sense amplifier in a DRAM device having a conventional open bit line structure and FIG. 2 is a cross-sectional view taken along line I-I′ and line II-II′ shown in FIG. 1.
Referring to FIGS. 1 and 2, an island-shaped active region and an isolation region are formed in a substrate on which a sense amplifier is formed. Two transistors connected in series are formed in the active region.
A unit sense amplifier A, which receives voltage signals from a bit line 14a and a bit line bar 14b and amplifies the voltage signals, thereby reading data in a selected cell, includes a first transistor 12a formed in a first active region 10a and a second transistor 12b formed in a second active region 10b. 
In the unit sense amplifier A, the bit line 14a is electrically connected to a first gate structure 16 of the first transistor 12a and a second source region of the second transistor 12b. Additionally, the bit line bar 14b is electrically connected to a second gate structure 18 of the second transistor 12b and a first source region of the first transistor 12a. The above electrical connections may be made through gate plugs 20 and 22 and source plugs 24a and 26a that are connected to the gate structures 16 and 18 and the source regions of the first and second transistors 12a and 12b, respectively. The bit line 14a and the bit line bar 14b make contact with top surfaces of the gate plugs 20 and 22 and the source plugs 24a and 26a. 
Additionally, drain plugs 24b and 26b are formed in drain regions of the first and second transistors 12a and 12b, and a wiring 15 is formed to be electrically connected to the drain plugs 24b and 26b. 
The unit sense amplifier A selectively drives each transistor electrically connected to the unit sense amplifier A using the voltage signals received from the bit line 14a and the bit line bar 14b so that a difference between the voltage signals of the bit line 14a and the bit line bar 14b may be amplified.
However, a difference between the voltage signals received from the bit line 14a and the bit line bar 14b is small. As a result, it is difficult to amplify the differences between the voltage signals of the bit line 14a and the bit line bar 14b. Thus, an operational defect in which data output from the bit line 14a and the bit line bar 14b are switched, or in which the bit line 14a and the bit line bar 14b always output the same data, may be generated. The above operational defect of the transistors and/or the unit sense amplifier A may occur due to misalignment in a photolithography process.
As shown in FIG. 2, when misalignment occurs in a photolithography process for forming the source and drain plugs 24a, 26a, 24b and 26b, which are electrically connected to the source and the drain regions of the first and second transistors 12a and 12b, respectively, completed source and drain plugs 24a, 26a, 24b and 26b may partially contact isolation regions. As a result, a contact area of each of the plugs 24a, 26a, 24b and 26b with corresponding active regions may vary.
When the contact area of each of the plugs 24a, 26a, 24b and 26b with corresponding active regions varies, a resistance between the plugs 24a, 26a, 24b and 26b and the bit line 14a and the bit line bar 14b may vary. Thus, signal inputs may be delayed in the bit line 14a or the bit line bar 14b having a relatively high resistance so that output signals from the bit line 14a or the bit line bar 14b may be switched or the output signals may always have a constant value of either “1” or “0.” Consequently, misalignment occurring in photolithography processes used in forming plugs 24a, 26a, 24b and 26b should be reduced to reduce operational defects such as those mentioned above. However, misalignment margins become very narrow as design rules become gradually reduced. Therefore, there are limits as to how much misalignment can be reduced.